The aims of this research are to determine the biogeographic distribution of Leishmania mexicana, to further clarify the role of woodrats (Neotoma spp.) as reservoirs and sand flies (Lutzomyia spp.) as enzootic vectors, and to determine environmental factors that influence transmission. This information is essential to prevent the emergence or introduction of leishmaniasis in the United States, and to reduce transmission to humans in countries such as Nicaragua, where severe outbreaks are presently occurring. Strengths of this study are that it is highly interdisciplinary (ecology, biogeography, mammalogy, entomology, parasitology, modeling, molecular biology, evolutionary biology), and studies will be conducted at several spatial and temporal scales.